1. Field of the Invention
The present invention relates to an exposure mask, and particularly relates to optimal conditions of phase difference and transmittance of an exposure mask.
2. Description of the Related Art
Note that in this specification, a “semiconductor device” refers to any device that utilizes semiconductor characteristics to function, and electro-optical devices, semiconductor circuits and electronic devices are all included in the category of the semiconductor device.
An active matrix type liquid crystal or an EL display technique of utilizing thin film transistors (TFTs) has been in the spotlight in recent years. An active matrix display is advantageous compared to a passive matrix display in response speed and contrast.
For such a semiconductor device having a display, various TFTs of which roles are different have been used depending on its use application and function; an LDD (Lightly Doped Drain) structure which realizes a small off-current value, a GOLD (Gate Overlapped LDD) structure having a region where a gate electrode and an LDD region are overlapped, which is superior as a measure against hotcarriers, a single-drain structure, and the like have been used.
Conventionally, an LDD region and a GOLD region have been formed in a self-aligned manner with a gate electrode as a mask, which is advantageous in that increase in the number of photolithography steps can be suppressed.
In case, the formation of an LDD structure and a GOLD structure is performed by using only a process such as dry etching; thus, a single drain structure, an LDD structure, and a GOLD structure cannot be formed separately for each circuit.
Further, in the case of forming an LDD region by using a step of forming a spacer (also called a sidewall) on a side wall of a gate electrode, the manufacturing steps would become complex.
As a method for solving the above problems, a photomask or a reticle provided with an assistant pattern (a halftone film) having a function of reducing light intensity, which is made of a semi-transparent film is used in a photolithography step of forming a gate electrode, thereby transistors having an LDD structure, a GOLD structure, and a single-drain structure are formed for respective circuits (reference 1: Japanese Patent Laid-Open No. 2002-151523). Note that such a photomask or a reticle provided with a halftone film is called an exposure mask (a halftone mask).
In the case where a resist for obtaining a desired gate electrode by using the above-mentioned halftone mask, shape of the resist depends on the phase difference and the transmittance with respect to exposure light, of the halftone mask. Therefore, the transmittance and the phase difference of exposure light are required to be controlled.
For example, a shape of a resist required for obtaining a gate electrode with which an LDD region can be formed in a self-aligned manner is a shape where a central portion of the resist has a region with a certain thickness, a region including both end portions of the resist has a region with a smaller thickness than the central portion, and both the end portions have gentle slope.
FIG. 12 shows a resist shape manufactured by using a halftone mask formed in such a condition that the transmittance of i-line (365 nm), n=0.2 (or also defined as T=20%), and the phase difference of i-line, (Δθ)=130°. As indicated by arrows, both end portions of the resist are convex. When a conductive layer is etched using the resist shown in FIG. 12 to form a gate electrode, the conductive layer becomes thick locally under the convex portion. As a result, in a doping step to a semiconductor layer which is performed after the formation of the gate electrode, the carrier concentration in a semiconductor layer provided in a layer under the locally-thick conductive layer is locally changed.
The following is considered as a reason thereof; exposure light which transmits through a transparent region of the halftone mask and exposure light which transmits through a halftone film (semi-transparent region), interfere with each other, and intensity of the exposure light which transmits through a boundary portion of the halftone film is decreased, thereby a convex portion is formed in an end portion of the resist (boundary portion of the halftone film).